The invention relates to medical diagnostic equipment, and more particularly relates to equipment which uses ultrasound to diagnose a patient's medical condition. In its most immediate sense, the invention relates to ultrasound blood flow monitors.
In conventional ultrasound blood flow monitors, the velocity of the patient's blood flow is determined using the Doppler effect. In such apparatus, an insonifying sound wave is reflected back by the patient's blood. According to the Doppler effect, the frequency shift in the reflected-back wave relative to the insonifying wave (the "Doppler shift") is determined by the velocity of the patient's blood flow, and conventional blood flow apparatus measures this frequency shift so the diagnostician can know the direction and speed with which the patient's blood is flowing.
Because the Doppler shift is quite small as compared with the frequency of the insonifying sound wave, it cannot be determined accurately on the basis of only one reflected-back sound wave. A plurality of such reflected-back sound waves must be considered, and it is necessary to compute the autocorrelation of the Doppler shift using all the reflected-back signals to arrive at an accurate estimate of the patient's blood flow velocity. Such computation requires substantial computer resources.
It would be advantageous to provide alternate ultrasound method and apparatus for measuring the velocity of a moving scatterer (e.g. the patient's blood stream) which would not require intensive computation (thereby reducing the need for high-performance computer resources).
The invention proceeds from the realization that as a series of insonifying sound waves are reflected back from a moving scatterer, such as the patient's blood stream, successively reflected-back sound waves are time-shifted as a function of the velocity of the scatterer. Thus, in accordance with the invention, the displacement of corresponding wavefronts in the reflected-back sound waves is determined and used to compute the velocity of the scatterer. In accordance with the preferred embodiment of the invention, a feature in each of the reflected-back sound waves is identified and the movement of that feature is traced. By using a feature-tracing scheme, computation of scatterer velocity is greatly simplified.
Advantageously, apparatus in accordance with the preferred embodiment of the invention records in real time the "raw" ultrasound information accumulated by the transducer while in receive mode. This information is stored and scatterer velocity is determined periodically in real time each time sufficient data has been acquired. To do this, the data is processed by simple subtraction to remove information which relates to stationary scatterers. The remaining information relates to moving scatterers, such as the patient's blood. Advantageously, a computer search strategy can be used to extract velocity-related information; little if any computation is required.